Examination of the New α-(2‘Z-Fluoro)vinyl Trigger with Lysine Decarboxylase:  The Absolute Stereochemistry Dictates the Reaction Course

• Published in: Journal of the American Chemical Society 2007-01, Vol.129 (2), p.258-259
• Main Authors: Karukurichi, Kannan R, de la Salud-Bea, Roberto, Jahng, Wan Jin, Berkowitz, David B
• Format: Article
• Language: English
• Subjects: Carboxy-Lyases - chemistry; Enzyme Activation; Enzyme Inhibitors - chemical synthesis; Enzyme Inhibitors - chemistry; Hafnia alvei - enzymology; Lysine - analogs & derivatives; Lysine - chemical synthesis; Lysine - chemistry; Magnetic Resonance Spectroscopy - methods; Molecular Conformation; Sensitivity and Specificity; Stereoisomerism; Structure-Activity Relationship; Vinyl Compounds - chemical synthesis; Vinyl Compounds - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja067240k

The first examination of a terminal α-fluorovinyl trigger in an amino acid decarboxylase active site is reported. To investigate the enantiospecificity of inactivation with this new AADC trigger, an enantioselective synthesis of l-α-(2‘Z-fluoro)vinyllysine and its d-antipode has been developed. Control of stereochemistry is achieved through introduction of the amino acid side chain via alkylation of a chiral vinylglycine-derived dienolate. Facial selectivity is conferred by a trans-2‘(β-naphthyl)-2‘-propylcyclohexyl ester auxiliary, available in both antipodal forms (Comins protocol). The alkylation employs a new electrophile, N-p-methoxybenzyl-N-(2‘-trimethylsilylethanesulfonyl)-4-iodobutylamine, for convergent installation of the lysine side chain. Vinyl to 2‘-fluorovinyl interconversion then provides l- and d-α-(2‘Z-fluoro)vinyllysine in 97-99% ee, as demonstrated by chiral HPLC. Both time-dependent enzyme kinetics and 19F NMR reveal striking differences in the behavior of these two antipodes in the lysine decarboxylase active site. The l-antipode displays time dependent inactivation (t 1/2 = 3 ± 1 min; K I = 86 ± 22 μM), whereas the d-antipode behaves as a substrate, being completely turned over to α-(2‘Z-fluoro)vinylcadaverine. Titration of LDC with varying amounts of l-α-(2‘Z-fluoro)vinyllysine provides an estimate of 20 ± 3 for the partition ratio for this antipode. 19F NMR provides a more detailed account of the inactivation with the l-antipode, revealing that 1 in 3.4 turnovers of this mechanism-based inhibitor results in errant protonation (required by design), with 1 in 5 errant protonation events leading to LDC inactivation. This gives an overall partition ratio of 16 ± 2. Fluoride-selective electrode measurements are in agreement with 19F NMR estimates of [fluoride] released.